1. Field of the Invention
The invention relates to an ink-jet device and a method for driving the ink-jet device.
2. Description of Related Art
Impact printers have been giving way to non-impact printers, which are now remarkably expanding the market. Of the non-impact printers, an ink-jet printer is the most noticeable because of its simple principle and facility of displaying data in multiple gradation and in full color. Of the ink-jet printers, the drop-on-demand type which projects only droplets of ink to be used for printing characters is rapidly achieving widespread use due to good ink projection efficiency and low running cost.
As representative drop-on-demand type ink jet printers, there have been disclosed a Kyser type in U.S. Pat. No. 3,946,398 and a thermal-jet type in Japanese unexamined Patent Publication No. 55-27282. These types, however, have such very difficult problems that the former is hard to miniaturize, while the latter is required to use heat-resistant ink because heat is applied to the ink.
For simultaneous solutions to the above-described problems, newly proposed is a device utilizing a modification of piezoelectric ceramic shear mode that has been disclosed in U.S. Pat. No. 4,879,568.
As shown in FIGS. 1 and 2, the ink-jet device 600 using the piezoelectric ceramic shear mode and a method for driving the ink-jet device comprises a bottom wall 601, a top wall 602, and shear mode actuator walls 603 disposed therebetween. The actuator walls 603 each comprise a lower wall 607 attached to the bottom wall 601 and polarized in the direction of an arrow 611, and an upper wall 605 attached to the top wall 602 and polarized in the direction of an arrow 609. The actuator wall 603 serves in pairs, between which an ink flow passage 613 is formed; and between adjacent pairs of actuator walls 603 is formed a space 615 which is narrower than the ink flow passage 613.
In one end of the ink flow passages 613 a nozzle plate 617 having nozzles 618 is secured, and on both sides of each actuator wall 603 are provided electrodes 619 and 621 as metal layers respectively. Specifically, on the actuator wall 603 on the ink flow passage 613 side the electrode 619 is provided, and on the actuator wall 603 on the space 615 side the electrode 621 is provided. The surface of the electrode 619 is covered with an insulating layer 630 for insulation from the ink. The electrode 621 facing the space 615 is connected to a ground 623, while the electrode 619 provided in the ink flow passage 613 is connected to a driving circuit 625 which gives an actuator driving signal.
Next, the method of manufacturing the ink-jet device 600 will be explained. First, a piezoelectric ceramic layer polarized in the direction of the arrow 611 is attached to the bottom wall 601, and a piezoelectric ceramic layer polarized in the direction of the arrow 609 is glued to the top wall 602. The thickness of each piezoelectric layer is equal to the height of the lower wall 607 and the upper wall 605. Next, parallel notches are formed in the piezoelectric ceramic layer by utilizing the rotation of a diamond cutting disk, thereby forming the lower wall 607 and the upper wall 605. The electrodes 619 and 621 are formed by a vacuum evaporation process on the sides of the lower wall 607 respectively. On the electrode 619 is provided the insulating layer 630. In a similar manner the electrodes 619 and 621 are formed on the sides of the upper wall 605 respectively, and on the electrode 619 the insulating layer 630 is provided.
The peaks between the notches of the upper wall 605 and the zenithal section of the lower wall 607 are bonded together to define the ink flow passages 613 and the spaces 615. Next, the nozzle plate 617 forming the nozzles 618 is bonded to one end of the ink flow passage 613 and the space 615 so that the nozzle 618 will correspond to the ink flow passage 613, and the other end of the ink flow passage 613 and the space 615 are electrically connected to the driving circuit 625 and to the earth ground 623.
Then the electrode 619 of each ink flow passage 613 is applied with a voltage from the driving circuit 625, whereby each actuator wall 603 undergoes a piezoelectric thickness deformation in a direction such that the volume of the ink flow passage 613 will increase.
FIG. 3 shows one example of the piezoelectric thickness deformation. When a specific voltage E (V) is applied to an electrode 619C of an ink flow passage 613C, there is produced an electric field in actuator walls 603E and 603F in the direction of arrows 629 and 630 respectively, causing the actuator walls 603E and 603F to undergo the piezoelectric thickness deformation in the direction in which the volume of the ink flow passage 613C will increase. At this time the pressure in the ink flow passage 613C including the vicinity of the nozzle 618C will decrease. This condition is maintained for a period of time T for one-way propagation of the pressure wave in the longitudinal direction within the ink flow passage 613. Then, the ink is supplied during this period of time from a common ink chamber 626 into the ink flow passage 613.
The time T for one-way propagation stated above is the time required by the pressure wave in the ink flow passage 613 to propagate longitudinally in the ink flow passage 613; T=L/a is determined by the length L of the ink flow passage 613 and the speed of sound "a" in the ink flowing in the ink flow passage 613. According to the pressure wave propagation theory, when the time T after the application of the voltage has passed, the pressure in the ink flow passage 613 turns to a positive pressure. The voltage being applied to the electrode 619C of the ink flow passage 613C is reset to zero in accordance with the inversion.
Then, the actuator walls 603E and 603F return to the condition before the deformation shown in FIG. 1, applying a pressure to the ink. Thus when the pressure that has turned into the positive pressure and the pressure established by the recovery of the actuator walls 603E and 603F to the condition before the deformation are combined, a relatively high pressure is established in the vicinity of the nozzle 618C of the ink flow passage 613C, ejecting the ink from the nozzle 618C.
However, if the ink is ejected at one time from a plurality of adjacent ink flow passages 613, a peak current increases, and a voltage drop occurs in the wiring, resulting in a lowered driving voltage and giving an adverse effect to ink ejection. Also, it becomes necessary to use a larger-diameter power source wiring in the driving circuit, which will increase the size and cost of the circuit.
To solve these problems, the ink-jet device was driven with the phases of the driving voltage signals to be applied to the actuator walls 603 corresponding to the plurality of adjacent ink passages 613 mutually shifted. As shown in FIG. 4, a quick-phase first driving signal 510 and a slow-phase second driving signal 520 are shifted by D in phase though the driving voltage amplitude E (V) and the pulse width WC are common.
However, the method for driving the ink-jet device of the above-described constitution has a problem that since the ink-jet device is driven with the phases of the driving voltage signals to be applied to the actuator walls 603 deviated in relation to the plurality of adjacent ink flow passages 613, the target position on paper of ink droplets ejected being deviated by the amount of phase shift, resulting in a deteriorated printing quality.
Deviation of the target position on paper of ink droplets will be explained by referring to a graph shown in FIG. 5.
FIG. 5 shows the amount of deviation of the ink-jet target position when printing of 720 dpi resolution is done by ejecting the ink onto the paper placed 1 mm apart from the nozzle at a frequency of 10 kHz. It is understood that at an ink-jet velocity of 5 m/s, the amount of deviation of the target position varies to 3.5 .mu.m, 7.1 .mu.m and 10.6 .mu.m with the change of the phase difference of 10 .mu.s, 20 .mu.s and 30 .mu.s, respectively.
Furthermore, the deviation of the target position of ink droplets can be corrected by changing the ink-jet velocity to 5.25 m/s, 5.55 m/s and 5.9 m/s in accordance with a slow-phase driving voltage signal when the phase difference varies to 10 .mu.s, 20 .mu.s, and 30 .mu.s; therefore the amplitude of the driving voltage signal is changed by the quick-phase driving voltage signal and the slow-phase driving voltage signal so that the ink droplets to be projected will simultaneously reach a recording medium (e.g., paper) notwithstanding the phase difference of the driving voltage signal, thus resulting in adverse effects of the necessity of two or more power sources and an increased manufacturing cost.